1. Field of the Invention
The present invention generally relates to integrated circuit (IC) proportional to absolute temperature (PTAT) temperature sensors, and more specifically to an IC temperature sensor with a programmable offset.
2. Description of the Related Art
The base-emitter voltage V.sub.be of a forward biased transistor is a linear function of absolute temperature T in degrees Kelvin (.degree.K.), and is known to provide a stable and relatively linear temperature sensor. ##EQU1## where k is Boltzmann's constant, T.sub.k is the absolute temperature (.degree.K.), q is the electron charge (k/q=86.17 .mu.V/.degree.K.), I.sub.c is the collector current, A.sub.e is the emitter area, and J.sub.s is the saturation-current density. PTAT sensors eliminate the dependence on collector current by using the difference .DELTA.V.sub.be between the base-emitter voltages V.sub.be1 and V.sub.be2 of two transistors that are operated at a constant ratio between their emitter-current densities to form the PTAT voltage. The emitter-current density is conventionally defined as the ratio of the collector current to the emitter size (this ignores the second order base current).
The basic PTAT voltage .DELTA.V.sub.be is given by: ##EQU2## The basic PTAT voltage is amplified so that its gain, i.e. its sensitivity to changes in absolute temperature, can be calibrated to a desired value, suitably 10 mV/.degree.K., and buffered so that a PTAT voltage can be read out without corrupting the basic PTAT voltage.
A drawback of standard PTAT sensors is that at ordinary operating temperatures for most ICs there is a large offset voltage signal. For example, if the desired operating range for an IC is 0.degree. to 125.degree. C. (273.degree. to 398.degree. K.) and the sensor has a gain of 10 mV/.degree.K., the PTAT sensor will have an offset voltage of 2.73 V at 0.degree. C. If the gain of the PTAT sensor is not perfectly stable, a relatively small change in the offset voltage may shift the output temperature by several degrees. To read out a temperature from 0.degree. to 125.degree. C., a reference voltage of precisely 2.73 V must be subtracted from the output of the PTAT sensor. Providing a reference voltage with adequate precision and stability is difficult and costly. Furthermore, PTAT sensors require relatively large supply voltages to supply the offset voltage in addition to the voltage needed to respond over the desired operating range and any head voltage needed to operate the sensor. Thus, products such as lap top computers which run off approximately 3 V supplies cannot use PTAT sensors.
Pease, "A New Fahrenheit Temperature Sensor," IEEE Journal of Solid-State Circuits, Vol. SC-19, No. 6, December 1984, pages 971-977, discloses a temperature sensor that provides an output voltage scaled proportional to the Fahrenheit temperature without subtracting a large constant offset voltage at the output. Pease generates a PTAT voltage using a conventional transistor pair and internally subtracts two base-emitter voltages to shift the PTAT voltage by a constant offset voltage. A non-inverting amplifier is used to multiply the shifted PTAT voltage by a fixed gain, e.g. 1.86, to simultaneously set the sensor's desired offset voltage, e.g. 770 mV at 77.degree. F., and gain, e.g. 10 mV/.degree.F. The gain is inherently calibrated by simply trimming the offset error at room temperature. In this manner, Pease effectively subtracts the offset voltage so that the sensor's output voltage is zero at 0.degree. F.
Pease's circuit topology has several drawbacks. The shifted output voltage is produced in two separate stages: a constant offset is first subtracted from the basic PTAT voltage and then the result is multiplied by the amplifier to achieve the desired output. This increases the sensor's complexity. Because the amplifier is used to buffer the output voltage in addition to providing gain, any errors in the amplifier such as offset voltage or offset voltage drift are reflected into the output voltage signal and may cause a temperature shift. For the Fahrenheit sensor to measure 0.degree. F., the inverting input of the amplifier must be able to go to ground potential. This type of amplifier is complex and difficult to design.
National Semiconductor Corporation produces an LM35 series of Precision Centigrade Temperature Sensors which are disclosed in their Data Acquisition Data Book, 1993, pages 5-12 to 5-15 and are the centigrade equivalent of Pease's Fahrenheit sensor. The centigrade sensors exhibit the same problems and require a minimum 4 V supply voltage.